The invention relates to a valve having a closure member which co-operates with a valve seat and is movable by an actuating device with a driver element, which driver element is guided in a housing.
U.S. Pat. No. 5,145,148 discloses such a valve, which is used in a brake system to control the flow, or the pressure, of the brake fluid. There, the driver element is in the form of a rod which, on actuation by an armature, which changes its position when current flows through a coil surrounding it, pushes a ball away from a valve seat. The rod is in this particular case surrounded by a nylon bushing which guides the rod in the housing. The rod is relatively thin in order not to disrupt the magnetic circuit. The nylon bushing is correspondingly thick.
In such a construction, malfunctioning of the valve can occasionally be observed, that is, the valve does not open or does not close when desired, but remains in the position it has assumed. This is attributed to the different thermal expansion coefficients of driver element and housing on the one hand and of the nylon bushing on the other hand. If the nylon bushing expands to a relatively great extent, the driver element becomes jammed in the housing. This effect can be partly eliminated by providing a larger clearance between the nylon bushing and the housing, but this clearance leads to imprecise guiding. This measure is also not suitable if it is wished to guide the driver element in the housing with an improved seal, nor if a defined gap between the driver element and the housing is an important factor, the gap being intended to damp the movements of the driver element.
Such a gap also causes accelerated wear of the valve, because as the driver element moves, not only is there purely frictional loading on adjacent surfaces, but as the driver element vibrates in the housing there is also a possibility that impact loading will occur, to the detriment of the plastics material.